1. Field of the Invention
The present invention relates to an apparatus for joining a coated wire to a terminal member by supply of current.
2. Description of the Related Art
Coated wires are electric wires each made of a conductor coated with an insulator such as polyimide, polyurethane, enamel or vinyl. Hitherto known is a resistance welding apparatus capable of welding such a coated wire to a terminal member by mere supply of current without subjecting the coated wire to chemical or mechanical pretreatment. This type of resistance welding apparatus comprises a welding head that includes an intermediate electrode interposed between the coated wire and an upper electrode abutting against the coated wire, the intermediate electrode forming an electric bypass leading to a lower electrode abutting against the terminal member.
In such a configuration, the insulator of the coated wire shuts out the current at the initial stage of the supply of current, so that the current flows from the upper electrode through the intermediate electrode bypassing the lower electrode. As a result, the upper and intermediate electrodes (especially, the contact portion between the two electrodes) generate heat by Joule effect, which in turn apply thermal energies to the coated wire. Finally, the insulating film of the coated wire melts and the interior conductor or bare wire become exposed. After the exposure of the coated wire conductor in this manner, the workpieces (coated wire and terminal member) form a part of the electrically conductive path, allowing current to flow between the upper electrode and the lower electrode. The current passing though the workpieces causes the work contact portions to generate heat by Joule effect and fuse and metallurgically join together. Thus, by merely passing current through the welding head, the coated wire insulator is automatically removed so that the coated wire conductor is resistance welded to the terminal member.
Such a conventional resistance welding apparatus has managed (controlled) the main current flowing through the welding head (especially, the upper electrode) and the current-supplying time to have certain values without distinguishing the current used for the melting removing of the coated wire insulator from the current used for the workpiece resistance welding.
However, in spite of a series of steps associated with the same coated wire within the continuous current-supplying time, melting and removing of the insulator and the conductor resistance welding are intrinsically different steps, which are naturally to be given separate optimum current values. In addition, the current-supplying time should also consist of two stages for separate control.
Any means has not been found so far that monitors the current-supplying time (first current-supplying time) required for melting and removing of the insulator, which has made it difficult to manage the current-supplying time (second current-supplying time) for resistance welding to have a desired set value. For this reason, the second current-supplying time and thus the joint quality were influenced by any possible variance of the first current-supplying time. More specifically, too long a first current-supplying time results in too short a second current-supplying time, which impedes the acquisition of a sufficient welding strength. Inversely, when the second current-supplying time becomes too long as a result of short first current-supplying time, there has arisen a drawback that the workpiece joining portions, especially, the coated wire conductor may excessively fuse, which is apt to cause a defective welding due to occurrence of spatters.